Detection of wear in electrical conductors such as insulated wires is critical to avoid simple inconvenience and, in some instances, catastrophic system failure. Electrical cables typically consist of a conductor, such as a metallic wire or a plurality of wires, that is insulated with a jacket or sheath around the circumference of the wire. The insulating material can be plastic, rubber, and the like. Power chains have either relatively flat or cylindrical, elongated structures that consist of a series of long, parallel conductors that are separated from one another by an insulating material.
Power chains are incorporated into moving machinery such as robotics and are subject to fatigue as they undergo cyclic motion, as well as by abrasion, impact or tension overload. There exists a need to determine wear in electrical conductors and in particular those that are subject to constant frictional forces such as continuously moving cables. Aside from programmed maintenance and inspection, the most common method of detecting cable wear entails deploying one or more “wear conductors” along strategic positions in the cable that appear to be subject to the most severe wear. Wear can be inferred to occur when an electrically open circuit from a broken conductor is detected. See, for example, U.S. Pat. No. 6,034,531 to Senglat et al. and U.S. Pat. No. 6,215,315 to Mitsuru Maejima, WO 01/84117 to Pierre et al. and WO 01/42802 to Harry Orton. A major shortcoming of this approach is that the sensing of broken conductors does not lend itself to monitoring progressive wear. Some cable manufacturers also measure cable resistance during life cycle testing; in most cases, this is a static procedure that employs a basic ohmmeter and that is performed when the test equipment is shut down. Finally, prior art methods that endeavor to predict cable wear by measuring resistance do not adequately compensate for temperature fluctuations in the cables being monitored.
U.S. Pat. No. 6,777,948 to Dominelli et al. describes a method of detecting wear in components of high voltage electrical equipment by embedding a tracer material that emits electromagnetic radiation into the components at predetermined depths therein. As the components degrade, the tracer material can be detected when it is exposed to an electrical arc. Introducing tracer materials is not practical for many applications and can actually interfere with the operations of many electrical conductors.
U.S. Pat. No. 5,015,859 to Wayne H. Uejio describes a method of detecting wear of electrical conductors such as insulated wires by wrapping an optical fiber around the article and measuring the transmission of light through the optical fiber. Increased transmission loss is said to indicate wear of the article. Incorporating optical fibers, which are very fragile, into moving power chains is not feasible for many applications and would be cost prohibitive.